1. Field of the Invention
The present invention relates to systems and methods for data transmission, and more particularly, to data transmission system and method for exchanging packets including data channels.
2. Description of the Related Art
Generally, in cellular mobile communications, packets are transmitted by means of adaptive radio link control including control operations such as adaptive modulation, retransmission control and scheduling, in order to raise the transmission efficiency of data packets. These control operations are performed using a control channel transmitted simultaneously with a data channel, and a base station uses the control channel of a downlink (link from the base station to a mobile station) to notify a mobile station of the radio link parameters used in the data channel. In the case of the adaptive modulation, for example, the control channel carries information about the modulation type and encoding rate of the data channel. In the case of the retransmission control, the control channel carries information about the packet number of a packet being transmitted via the data channel, a retransmission count, etc., and in the case of the scheduling, the control channel conveys information about the mobile station (user) ID simultaneously with a packet.
The adaptive radio link control is performed in units of wireless resources such as time, frequency, and space (antenna or directional beam). The mobile station measures a parameter indicative of transmission channel quality, which is called CQI (Channel Quality Indicator), with respect to each of the wireless resources by using a pilot channel of the downlink, and feeds back the information to the base station by using the control channel of an uplink (link from the mobile station to the base station) (e.g., Unexamined Japanese Patent Publication No. 2004-135287). This Patent Document 1 proposes, in addition to the periodic feedback of the CQI, a method of feeding back the CQI when NACK (Negative Acknowledgment) is returned. In Patent Document 1, the CQI is not fed back when ACK (Acknowledgment) is returned.
For the CQI, SIR (Signal-to-Interference power Ratio) is usually employed. SIR is calculated using a channel estimation value obtained from the pilot channel of a downlink, in the manner described below.
Provided the channel estimation value obtained from a k-th pilot is hk, received power S, interference power I and SIR are obtained according to the following equations (1) to (4) using K pilot symbols:
                    S        =                              1            K                    ⁢                                    ∑                              k                =                1                            K                        ⁢                                                                            h                  k                                                            2                                                          (        1        )                                m        =                              1            K                    ⁢                                    ∑                              k                =                1                            K                        ⁢                          h              k                                                          (        2        )                                I        =                                            1              K                        ⁢                                          ∑                                  k                  =                  1                                K                            ⁢                                                                                                            h                      k                                        -                    m                                                                    2                                              =                                                    1                K                            ⁢                                                ∑                                      k                    =                    1                                    K                                ⁢                                                                                                h                      k                                                                            2                                                      -                          m              2                                                          (        3        )                                SIR        =                  S          /          I                                    (        4        )            
In next-generation radio communication systems, the time domain is segmented into wireless resource units called packets, and control operations such as adaptive modulation and scheduling are performed with respect to individual packets. Also, next-generation radio communication systems adopt multi-carrier transmission called OFDMA (Orthogonal Frequency Division Multiple Access). In OFDMA, wireless resources are segmented into frequency units (subcarriers), and control operations such as adaptive modulation and scheduling are performed with respect to individual frequencies. Further, next-generation radio communication systems use spatially multiplexed transmission called MIMO (Multiple-Input Multiple-Output) in which a plurality of transmit/receive antennas are used for transmitting/receiving data independently of one another. In this case, control operations such as adaptive modulation and scheduling can be performed with respect to each antenna.
The units into which wireless resources are segmented are used in combination. Namely, wireless resources are segmented in the time domain into packets and, in the case of OFDMA, are segmented also in the frequency domain. Further, where MIMO is adopted, wireless resources are segmented with respect to the individual antennas. By segmenting wireless resources into small units in this manner, it is possible to optimize the control operations. Also, in the case of scheduling a plurality of mobile stations (users), higher user diversity gains can be obtained by segmenting wireless resources into smaller units.
Thus, in next-generation radio communication systems, the adaptive radio link control is performed with respect to individual wireless resource units, and in order to optimize the control operations and to obtain high user diversity gains, it is necessary that wireless resources should be segmented into small units.
However, if wireless resources are segmented into small units, the number of CQIs to be measured increases, which leads to an increase in the amount of CQI information to be fed back. As a result, the load on the uplink used to feed back such information to the base station increases, giving rise to the problem that the usage efficiency of the uplink lowers.